Various types of flow meters are currently in use for measuring a volume flow of a fluid, such as a liquid or a gas, through a pipe. Ultrasonic flow meters are either Doppler flow meters, which make use of the acoustic Doppler effect, or travel time flow meters, sometimes also called transmission flow meters, which make use of a propagation time difference caused by the relative motion of source and medium. The travel time is also referred to as time of flight or transit time.
An ultrasonic travel time flow meter evaluates the difference of propagation time of ultrasonic pulses propagating in and against flow direction. Ultrasonic flow meters are provided as in-line flow meters, also known as intrusive or wetted flow meters, or as clamp-on flow meters, also known as non-intrusive flow meters. Other forms of flow meters include Venturi channels, overflow sills, radar flow meters, Coriolis flow meters, differential pressure flow meters, magnetic inductive flow meters, and other types of flow meters.
When there are irregular flow profiles or open channels, more than one propagation path may be necessary to determine the average flow speed. Among others, multi-path procedures are described in hydrometric standards such as IEC 41 or EN ISO 6416. As a further application, ultrasonic flow meters are also used to measure flow profiles, for example with an acoustic Doppler current profiler (ADCP). The ADCP is also suitable for measuring water velocity and discharge in rivers and open waters.